Wireless networks have been given attention as a system to escape from wiring according to conventional cable communication methods. For example, wireless LAN standards, such as IEEE (The Institute of Electrical and Electronics Engineers) 802.11a, IEEE802.11b, and IEEE802.1g, are typical. According to a wireless LAN, flexible Internet connection can be done, and not only existing cable LANs can be replaced but also Internet connection means can be provided even in public places such as hotels, airport lounges, train stations, cafés, or the like. Wireless LANs have already come into widespread use, and a wireless LAN function has been commonly implemented in not only information equipment such as personal computers (PC) or the like but also in CE (Consumer Electronics) devices such as digital cameras, music players, and so forth.
In order to configure a LAN using wireless technology, a method has commonly been used wherein a single apparatus serving as a control station called “access point (AP)” or “coordinator” is provided within an area, and a network is formed under the central control of this control station. The control station adjusts the access timings of multiple terminal stations within the network, and executes synchronous wireless communication wherein the respective terminal stations are synchronized mutually.
Also, Ad-hoc communication has been devised as another method for configuring a wireless network, wherein all the terminal stations are on an equal, and operate with peer-to-peer in an autonomous distributed manner, the terminal stations themselves determine access timing. Particularly, with a small wireless network configured of a relatively few clients positioned nearby, Ad-hoc communication can be conceived to be suitable wherein arbitrary terminals can execute asynchronous wireless communication directly without using a particular control station.
For example, networking with IEEE802.11 is based on the concept of BSS (Basic Service Set). The BSS is configured of two types of a BSS defined in “infrastructure mode” where there is a control station, and an IBSS (Independent BSS) defined in “Ad-hoc mode” where a network is just configured of multiple MTs (Mobile Terminal: mobile station or terminal station).
First, description will be made regarding the operation at the time of the infrastructure mode according to IEEE802.11.
Under the infrastructure mode, an AP integrates a range where electric waves arrive in the vicinity of the self station as a BSS, thereby configuring what is called a “cell” with a so-called cellular system. Terminal stations (MTs) in the vicinity of the AP are accommodated within the AP, and join a network as a member of this BSS. Specifically, the AP transmits a control signal called a beacon at an appropriate time interval, and an MT which can receive this beacon recognizes that the self station is in the vicinity of the AP, and further executes connection establishment with the AP thereof.
FIG. 16 illustrates an operation example of IEEE802.11 at the time of the infrastructure mode. With the example shown in the drawing, a communication station STA0 operates as an AP, and other communication stations STA1 and STA2 operate as an MT. The communication station STA0 serving as the AP transmits a beacon (Beacon) at a certain time interval, such as shown in the chart on the right side in the drawing. The AP internally manages the transmission interval of a beacon as a parameter called target beacon transmission time (TBTT: Target Beacon Transmit Time), and activates a beacon transmission procedure whenever time reaches the TBTT. Also, the beacon annunciated from the AP includes a beacon interval field, and the surrounding MTs can recognize the next beacon transmission time TBTT from this beacon interval field and the reception time of the beacon thereof.
Here, the BSS proceeds to a power-saving mode (PowerSave) as appropriate, where each MT executes a reception operation only in an intermittent manner, whereby low power consumption can be realized. Under the power-saving mode, at least a part of the MTs within the BSS operate in the sleep mode, and alternately enter into an Awake state where the transmitter and receiver are operated, or a Doze state where the power of the transmitter and the power of the receiver are turned off. The MTs can recognize the next beacon transmission time from the received beacon, and accordingly, under the sleep mode, when reception does not have to be executed, the MTs may turn off the power of the receiver to enter into a power-saving state until the next TBTT or the TBTT of multiple times ahead. The AP manages the timing where each MT in a sleep state awakes, in a central manner, and executes frame transmission to the MT in accordance with the timing of an Awake state, thereby assisting a power-saving operation. Note that an MT not in the sleep mode is called an active mode, where the transmitter and receiver are operated constantly (see FIG. 17).
The AP sometimes transmits broadcast data to the multiple MTs in a broadcasting manner. When at least a part of the MTs which communicates with the self station are under the sleep mode, the AP buffers transmission data internally at the time of executing broadcast transmission, and transmits this data immediately after transmitting a beacon. This cycle is informed with information included in the beacon, and each MT can recognize the TBTT when this broadcast data is transmitted. Upon recognizing the timing when the broadcast data is transmitted, the MT under the sleep mode awakes at this timing to operate the receiver. Thus, the broadcast data can be distributed to the MT under the sleep mode.
Subsequently, the operation at the time of the Ad-hoc mode according to IEE802.11 will be described.
With the Ad-hoc mode (IBSS) according to IEEE802.11, at the time of the multiple MTs recognizing the presence of each other, an IBSS is defined autonomously. These MT groups determine a TBTT with a certain interval. The transmission interval of a beacon is notified with a parameter within a beacon signal, and once the beacon signal is received, each MT can calculate the next TBTT. Subsequently, upon referencing a clock within the self station to recognize that the TBTT has come, each MT transmits a beacon after delay of random time in the case of recognizing that none of the MTs has transmitted a beacon. The MT which can receive this beacon can join this IBSS.
FIG. 18 illustrates an operation example of IEEE802.11 at the time of the Ad-hoc mode. The example shown in the drawing illustrates a situation wherein two communication stations STA1 and STA2 which operate as an MT make up an IBSS. In this case, one of the MTs belonging to the IBSS transmits a beacon each time the TBTT arrives. Also, there is a case where the beacons transmitted from the MTs collide.
With IEEE802.11, the power-saving (PowerSave) mode is stipulated with regard to IBSS as well, and the MTs can enter into a Doze state wherein the power of the receiver is turned off as appropriate. A predetermined time zone from a beacon transmission time TBTT is defined as an ATIM (Announcement Traffic Indication Message) Window. All the MTs belonging to the IBSS are in an Awake state until the period of the ATIM Window expires, and an MT which basically operates in the sleep mode can receive a beacon during this time zone. Subsequently, the MT can enter into a Doze state until the next beacon transmission time TBTT since completion of the ATIM Window.
In the case of the self station having information addressed to some station, each MT transmits an ATIM packet to the above communication partner during the time zone of this ATIM Window, thereby informing the reception side that the self station holds transmission information. On the other hand, the MT which has received the ATIM packet does not proceed to a Doze state until the reception from the station which transmitted the ATIM packet is completed, and operates the receiver continuously.
FIG. 19 illustrates an operation example in the case that there are three MTs of STA1, STA2, and STA3 within an IBSS. Upon the TBTT arriving, each MT of the STA1, STA2, and STA3 operates the backoff timer while monitoring a medium state for random time. With the example shown in the drawing, the timer of the STA1 expires earliest, and transmits a beacon. The STA1 has transmitted a beacon, and accordingly, the STA2 and STA3 which have received this do not transmit a beacon.
Now, let us say that the STA1 holds information addressed to the STA2, and the STA2 holds information addressed to the STA3. In such a case, the STA1 transmits a beacon, and then the STA2 receives the beacon, and then each of the STA1 and STA2 operates the backoff timer while monitoring a medium state for random time again. With the example shown in FIG. 19, the backoff timer of the STA2 expires earlier, and accordingly, first, an ATIM message is transmitted from the STA2 to the STA3. Upon receiving this ATIM message, the STA3 waits for just a short frame interval (SIFS: Short Inter-Frame Space), and then replies to the STA2 an ACK (Acknowledge) packet indicating that the ATIM message has been received. Upon transmission of the ACK from the STA3 being completed, the STA1 further operates the backoff timer while monitoring a medium state for random time, and upon the timer thereof expiring, the STA1 transmits the ATIM packet to the STA2. Subsequently, after the SIFS elapses, the STA2 replies to the STA1 an ACK packet indicating that the ATIM packet has been received.
Upon such exchange between the ATIM packet and the ACK packet being executed within the ATIM Window, the STA3 operates the receiver to receive the information from the STA2, and also the STA2 operates the receiver to receive the information from the STA1, for the subsequent sections as well.
The STA1 and STA2, which hold transmission information, wait for just a distributed frame interval (Distributed Inter-Frame Space: DIFS) equivalent to the minimum time when the medium is in an idle state immediately after completion of the ATIM Window, and then operates the backoff timer while monitoring each medium state for random time. With the example shown in FIG. 19, the backoff timer of the STA2 expires earlier, and accordingly, a data frame addressed to the STA3 is transmitted previously from the STA2. Subsequently, after the SIFS elapses, the STA3 replies to the STA2 an ACK packet indicating that the data frame has been received.
After the transmission of the data frame is completed, the STA1 waits for just the DIFS, and then further operates the backoff timer while monitoring the medium state for random time again, and upon the timer thereof expiring, the STA1 transmits a data frame addressed to the STA2. Subsequently, after the SIFS elapses, the STA2 replies to the STA1 an ACK packet indicating that the data frame has been received.
With the above procedure, the MT which has not received an ATIM packet within the ATIM Window, and does not hold information addressed to some station, turns off the power of the transmitter and the power of the receiver until the next TBTT, whereby consumption power can be reduced.
Also, in addition to the Ad-hoc mode stipulated with IEEE802.11, development relating to a communication system has been done wherein each communication station having no relationship between a control station and a controlled station operates in an autonomous distributed manner.
For example, a wireless communication system has been proposed wherein the respective communication stations mutually transmit a beacon in which information relating to a network is described to construct a network, thereby constructing an autonomous distributed network having no relationship between a control station and a controlled station, and accordingly, the communication state and the like of another communication station can be determined with the beacon thereof sophisticatedly (e.g., see Patent Document 1). This wireless communication system will be described next.
FIG. 20 and FIG. 21 illustrate an operation example with the wireless communication system, and an example of a transmission/reception procedure between communication stations. As shown in FIG. 20, there are two communication stations of the STA1 and STA2 in a mutually communicable range as an MT to join the network, and each MT sets each corresponding TBTT, and transmits a beacon signal periodically. Each MT extracts the information of an adjacent MT, and accordingly, receives the beacon signal of each MT periodically as appropriate.
Also, let us assume here that the STA1 enters into the sleep mode wherein the power of the transmitter and the power of the receiver are turned off as appropriate, and an MT in the power-saving mode alternately enters into an Awake state wherein the transmitter and receiver are operated, or a Doze state wherein the power of the transmitter and the power of the receiver are turned off (similar to the above).
FIG. 21 exemplifies a situation wherein data transmission is executed from the STA2 to the STA1. The upper stage in the drawing illustrates a packet transmission/reception sequence between the STA1 and the STA2, and the lower stage in the drawing illustrates the operation states of the transmitter and receiver of the STA1 serving as a data reception destination (level high indicates an Awake state, and level low indicates a Doze state). Note that, when both of the transmitter and receiver are in a Doze state, the communication station thereof enters into a power-saving state, and when one of the transmitter and receiver is in an Awake state, the current time is included in a time zone when the communication station thereof is not in a power-saving state.
With the MTs, a reception period (Listen Period) made up of a certain time zone is provided after transmitting a beacon, and the receiver is operated during this period. Subsequently, when traffic addressed to itself was not received during this Listen Period, the MTs can turn off the power of the transmitter and the power of the receiver to proceed to a power-saving state. With the example shown in FIG. 21, after transmitting a beacon B1-0, the STA1 operates the receiver for a while, and the STA2 has transmitted a packet to the STA1 during this period, whereby the STA1 can receive this.
Information called TIM (Traffic Indication Map) is described in a beacon signal. The TIM is annunciation information regarding whether or not this communication station has information addressed to which station at present, and a beacon receiving station can recognize whether or not itself has to execute reception by referencing this TIM. Each MT receives the beacon signal of a surrounding MT periodically, analyzes this TIM, and upon confirming that there is no data addressed to itself, turns off the power of the receiver to enter into a sleep state, but upon confirming that there is data addressed to itself, enters into a state wherein this data is received rather than entering into a sleep state.
FIG. 21 exemplifies a case where, with the TIM of the beacon B2-1, the STA1 has been called from the STA2. The STA1 which has received the beacon thereof executes response answering the call (0). Further, upon the STA2 which has received the response confirming that the STA1 is in a receivable state, the STA2 transmits a packet addressed to the STA1 (1). The STA1 which has received this confirms that the packet has been received normally, and then transmits an ACK (2).
As described with reference to FIG. 19, and FIG. 20 and FIG. 21, at the time of transferring data to a communication station which is in a Doze state under the sleep mode, the communication station serving as a data transmission source has to pass through a predetermined procedure. On the other hand, a communication station which does not communicate with a communication station under the sleep mode does not have to pass through such a procedure.
With the present specification, let us say that capabilities to execute data transfer as to a communication station under the sleep mode which repeats a Doze state and an Awake state alternately through a predetermined procedure will be referred to as “power-saving assistance”. The power-saving assistance mentioned here is similar to capabilities wherein an AP in the infrastructure mode centrally manages the timing when each MT in a sleep state awakes, and executes frame transmission in accordance with the timing of each MT in an Awake state. On the other hand, a communication station having no power-saving assistance capabilities has difficulty in executing data transfer to a communication station under the sleep mode. Accordingly, even if a communication station itself has a power-saving function to repeat a Doze state and an Awake state alternately, if a communication partner has no power-saving assistance function, the communication station itself has difficulty in proceeding to the sleep mode (upon proceeding to the sleep mode, data communication is disabled). That is to say, with a pair of communication stations to execute data communication, in order for one of the communication stations to proceed to the sleep mode, the other communication station has to have a power-saving assistance function.
Therefore, as one methodology wherein each MT realizes a power-saving operation with a communication system under the Ad-hoc mode (or autonomous distributed type), an arrangement can be conceived wherein information indicating the attributes of a communication station regarding whether or not capabilities to communicate with a communication station under the sleep mode (power-saving assistance) are possessed, and whether or not capabilities to proceed to the sleep mode (or whether to intend to proceed to the sleep mode) are possessed, is included in a beacon signal whereby mutual notification is performed, to match communication station attributes with each other, thereby determining beforehand whether a communicable relationship can be realized (or whether to be able to proceed to the sleep mode).
Under a communication environment where there are two communication stations STA1 and STA2 such as shown in the left of FIG. 20, in the case that one of the communication stations intends to proceed to the sleep mode, there has to be confirmed with reception of a beacon beforehand that the other communication station serving as a communication partner provides “power-saving assistance”. FIG. 22 and FIG. 23 each illustrate an example where matching of communication station attributes is executed.
With the example shown in FIG. 22, both stations display intent to enter into the sleep mode, but simultaneously, both stations provide “power-saving assistance”, and accordingly, the STA1 and STA2 match communication station attributes from the mutual beacon signal, whereby the STA1 and STA2 can enter into a communicable state.
On the other hand, with the example shown in FIG. 23, the STA2 displays intent to enter into the sleep mode, but the STA1 serving as a communication partner does not provide “power-saving assistance”, and accordingly, it is apparent that when the STA2 enters into the sleep mode, communication will break down. In such a case, the STA1 has difficulty in continuing communication, and accordingly, the STA1 may reject entering into a communication state with the STA2. Also, similarly, the STA2 has difficulty in proceeding to the sleep mode, and accordingly, the STA2 may reject entering into a communication state with the STA1.
Incidentally, with a wireless network, a terminal station serving as a communication partner is not necessarily accommodated in a range where the mutual electric waves arrive, so a great number of terminals are connected mutually with “multi-hop communication” wherein multiple communication stations transmit frames by relay. At present, standardization relating to multi-hop communication is proceeding as one task group within IEEE802.11. Within the present specification, let us say that a wireless network to execute multi-hop communication will be referred to as “mesh network”, and each communication station making up a mesh network will be referred to as “mesh point (MP)”.
The above Ad-hoc network wherein each communication station executes a communication operation in an autonomous distributed manner differs from a conventional fixed network, change in topology frequently occurs, so there has to be established a route control method, i.e., a routing protocol. For example, an on-demand routing protocol has been proposed by MANET (Mobile Adhoc NEtwork Working Group) of IETF (Internet Engineering Task Force) and others, wherein a route finding request is transmitted immediately before communication so as to create a route. As typical routing protocols, AODV (Adhoc On-demand Distance Vector), DSR (Dynamic Source Routing), TORA (Temporally Ordered Routing Algorithm), or the like can be exemplified. Any method of these basically has a procedure wherein creation of a route is started when receiving a data packet transmission request (route creation request), and communication is started when the route is created (i.e., transmission of a data packet is started).
As an example of a route setting procedure used for executing multi-hop communication, the function of the AODV will be described with reference to FIG. 24. In the drawing, three communication stations STA-A, STA-B, and STA-C are in a range where electric waves arrive mutually, but a situation is assumed wherein only the STA-D and STA-C are in a range where electric waves arrive.
With the AODV, for example, in the case that the STA-A attempts to transmit data to the STA-D, a procedure to check whether to transfer data in what kind of route is activated. First, the STA-A transmits a control packet called a route finding request message RREQ (Route Request) using broadcast, and waits for a route reply message RREP (Route Reply) from the STA-D which is a target station.
Information is described in the RREQ, such as the address of a route source station (STA-A in this case) and the address of a target communication station (STA-D in this case), a route metric value, and so forth. The route metric value mentioned here is a scale indicating the quality of a route or the like, and is represented with information accumulated so far from a route source station, for example, such as the number of hops, data rate, data rate×(1−packet error rate), or statistical data combined from these.
Upon confirming that the RREQ is a message of which the route target is not the self station, each of the STA-B and STA-C which have received the RREQ from the STA-A transfers this RREQ to a further communication station using broadcast again. At this time, a reversed route (Reverse Path) to the STA-A serving as a transmission destination of a route request message is set. The reversed route mentioned here means a route where a nearby terminal which transmitted the route request message is the next transfer destination in the case that a request occurs desiring to transmit data to the transmission source of the RREQ.
Subsequently, upon receiving the RREQ transferred from the STA-C, the STA-D confirms that the target of the RREQ thereof is the self station, and replies a RREP as to the STA-A. The RREP transmitted from the STA-D is received at the STA-C serving as a relay station, and further, the STA-C transfers the RREP to the STA-A.
The STA-A can recognize that data can be transferred to the STA-D via the STA-C by using such a route setting procedure, and simultaneously, the STA-C can extract information necessary as a relay station. The AODV has been exemplified here, but it goes without saying that, with a route setting procedure other than the AODV, it is common to set a route through exchange of broadcast control information.    Patent Document 1: WO2004/071022